This invention relates generally to devices controlling or conditioning the temperature of an environment in which electronic components are being electrically tested to determine their characteristics at selected or desired hot, cold and intermediate temperatures. Specifically, this invention relates to such devices that bathe the electronic component being tested in a stream of air or nitrogen gas at the selected or desired temperature.
Known devices direct a stream of temperature controlled air onto an electronic component, such as an integrated circuit, that is being tested or is having its electrical characteristics measured. Military standards require testing at -55.degree. C., 0.degree. C., +25.degree. C. and +150.degree. C. These known devices can control the air bathing the electronic component at temperatures from substantially -70.degree. C. to +200.degree. C. at substantially 10 SCFM. Typically, an air supplying cycle between -55.degree. C. and +125.degree. C. has required at least 30 to 45 seconds. This depended on whether the temperature was rising or falling, the mass, heat dissipation and volume of air flow involved. While the cycle times have been acceptable to date, they have slowed the automatic and manual testing of electronic components.
These temperature control devices comprise a rectangular cabinet movable on casters to desired locations in the factory or laboratory. Compressed air is supplied to the cabinet where it is filtered and dried. Alternatively, clean, dry nitrogen gas can be supplied. The clean, dry air or gas then passes through a mechanical refrigeration system where its temperature is lowered to the lowest possible temperature about -70.degree. C. The chilled air then is passed through a flexible tube about ten feet long to a nozzle and the electronic component under test. The tube includes along its length a heater element capable of heating the air or gas to well above the highest temperature desired or to about +200.degree. C. A shroud supports the nozzle above the electronic component and confines the temperature-controlled air around the electronic component. A thermocouple in the nozzle or on the electronic component controls the air temperature.
In these known devices, there is a substantial difference in the time required to change from supplying cold air to supplying hot air compared with the time required to go from supplying hot air to supplying cold air. Changing to supplying cold air is much slower than changing to supply hot air. The reason for this results from the temperature differential or delta T available for heating the air compared to that available for cooling the air.
The tube heater element substantially is a flexible metal tube through which electricity is passed to provide resistance heating of the tube material. The heat from the tube then is conducted into the air passing therethrough. The temperature available from this tube heater element can be much greater than the highest desired testing temperature of about +200.degree. C. Alternative heater elements such as quartz enclosed nickel-chromium filaments easily can produce over +1000.degree. C., if desired. This very great temperature differential provides a very quick change in temperature from cold to hot at a low heater element cost.
The refrigeration system used on the cold cycle, however, provides a much smaller temperature differential, typically a coldest possible temperature of -70.degree. C. for a low temperature test point of -55.degree. C., or a 15.degree. C. differential. The cost of increasing this differential on the cold side is prohibitive, one or two more stages of refrigeration would have to be added to the existing two stages of refrigeration.
Further, the chilled air itself is used to cool the full ten feet or so of tubing including the heating element. Air has a low heat capacity and, coupled with the small cold temperature differential, substantial time is required to cool the tube and nozzle to the desired low temperature. When seeking the desired hot temperature, however, large amounts of electrical power can be used resistively to obtain a desired hot temperature quickly.
These known systems or devices presently operate their refrigeration systems continuously at maximum cooling so there is no additional efficiency or cooling available. It is highly desireable, however, to achieve substantially decreased cycle times without substantially increasing the cost of the device with more refrigeration capacity.